On a MUP you ride to the right, obey the speed limit, and pass when safe.
Don't pass on a hill.
Sure the other rider could have ridden slower and been more courteous but in the end you were at fault.
Riding on a MUP is for the most part like driving but with 3 times the obstacles with a higher likelihood said obstacles will act in the most bizarre way possible.
I'm sorry but I do not understand what there is to not understand here.